Automobile body side molding and emblems are usually permanently mounted to exterior automobile surfaces either by mechanical fasteners or by pressure-sensitive adhesive tapes having a foamlike or foam core and relatively thin pressure-sensitive adhesive top layers or coatings. These exterior mounting tapes must not only strongly adhere to the protective and decorative automotive coatings and finishes, but must also withstand the environmental and mechanical stresses encountered by automobiles.
Typical foam and foamlike cores include filled or unfilled commercially available crosslinked polyethylene, polyurethane, polychloroprene, or acrylate foams. Acrylic copolymers, due to in part to their ease of formulation and weathering stability, are commonly the pressure-sensitive adhesives coated on such cores. These acrylic pressure-sensitive adhesives (PSAs) generally comprise a primary component of acrylate or methacrylate monomer or a combination of such monomers which, when polymerized, have a low glass transition temperature (Tg). These soft, tacky low Tg monomers are copolymerized with a secondary component consisting of high Tg monomers, usually polar monomers such as acrylic acid, methacrylic acid, itaconic acid, acrylamide, methacrylamide, and mixtures thereof. As described in Re U.S. Pat. No. 24,906 (Ulrich, assigned to 3M Company), when such polar monomers are incorporated with a predominance of low Tg monomers, a sufficiently tacky pressure-sensitive adhesive is formed having high cohesive or internal strength. Further increases in internal or cohesive strength (i.e., shear strength), which are often required to resist the severe environmental and chemical conditions found in automotive applications, can be obtained through crosslinking the acrylate PSAs.
These acrylate PSAs, most importantly, must adhere well to the high solids coatings employed by automotive manufacturers and suppliers. These coatings, formulated for appearance and hardness, are often acrylate or methacrylate copolymers containing hydroxy functional monomers which are subsequently crosslinked through the hydroxy functionalities in an acid catalyzed reaction using melamine curing agents. Due to the high melamine concentration of such coatings, they tend to have a basic character which enhances the adhesion of acrylate pressure-sensitive adhesives currently provided by automotive industry suppliers.
The acrylic-melamine crosslinked automotive coating formulations currently in use, while possessing the required toughness, appearance, and resistance to weathering, are subject to a new and problematic form of environmental degradation. Chemical attack of these coatings by acid-rain in industrialized regions causes undesirable, costly and often irreparable etching of such surfaces. As described by D. R. Bauer in an article entitled "Degradation of Organic Coatings. I. Hydrolysis of Melamine Formaldehyde/Acrylic Copolymer Films [Journal of Applied Polymer Science, 27(10), 3651-3662]," this coating degradation results from the irreversible acidolysis of the acrylic-melamine crosslinks and the formation of brittle melamine-melamine linkages.
In response to the problems resulting from acid-rain damage, new coating formulations having reduced or eliminated melamine curing agents have been developed. For example, European Patent Application 409,301 (Garden et al., assigned to Akzo N.V.) describes a high solids, high performance, urethane-linked automotive coating having enhanced acid etch resistance which comprises a particular polyurethane polyol and a hydroxyl groups-reactive crosslinking agent. In addition to the preferred aminoplast (i.e., melamine) crosslinkers, which could lead to the same acid susceptibility discussed above, Garden et at. also specify the use of polyisocyanate or blocked polyisocyanate crosslinking agents.
U.S. Pat. No. 5,137,972 (Cook, assigned to BASF) describes a two-component environmental etch resistant top coating comprising in one component at least one blocked isocyanate crosslinker and the second component a film forming polymer having a functionality which is reactive with the first crosslinking component. Cook, however, still allows for melamines as additional ingredients.
World Patent Appl. 92/05225 (Hazan et al., assigned to duPont) describes an automotive clear coating composition which is particularly resistant to etching caused by acid-rain. This composition, comprising a binder and liquid carrier, contains a binder comprising an acrylosilane polymer, an acrylic polyol polymer and an alkylated melamine formaldehyde crosslinking agent.
European Patent Application 450,963 (Okude et al., assigned to Nippon Paint Co.) describes a weather resistant automotive topcoating which eliminates the need for melamine-based curing agents and, unlike urethane-linked coatings, does not require toxic isocyanate starting materials. This ester-linked composition comprises a compound having both an epoxy and a hydroxy group combined with a copolymer prepared from a radically polymerizable monomer having an acid anhydride group (e.g., itaconic anhydride, maleic anhydride, citraconic anhydride, and the like) and a copolymerizable monomer. When the anhydride acid groups are substantially completely half esterified on such compounds, epoxy-acid linkages are formed, linkages which resist the degradation which cleaves current melamine-cured coatings when they are exposed to acid-rain.
Similarly, U.S. Pat. No. 4,732,790 (Blackburn et al., assigned to PPG Industries) describes a high solids automotive top coating composition comprising a low number average molecular weight polyepoxide, an anhydride curing agent, an onium salt curing catalyst. Such coatings, according to the inventors, exhibit high levels of solvent resistance, gloss, hardness and sag resistance, attributes typically and previously found only in high solids coatings using melamine curing agents.
Such ester-linked compositions, however, do not possess the same basic character as melamine-cured coatings. Thus, the acrylate pressure-sensitive adhesives currently in use do not adhere as well to these newer, less alkaline (i.e., slightly acidic) acid-rain resistant automotive coating compositions.
U.S. Pat. No. 4,943,461, (Karim, assigned to 3M Company), discloses a photopolymerized pressure-sensitive adhesive with outstanding ability to bond to polyvinylchloride (PVC). The adhesive comprises a copolymer of an alkylacrylate and a polar nitrogen containing vinyl monomer to which is added an acrylonitrile butadiene rubber. The adhesive is specifically designed for PVC applications and no reference is made to adhesion to automotive paints.
U.S. Pat. No. 4,946,742 (Landin, assigned to 3M Company), discloses normally tacky and pressure-sensitive adhesives having excellent long-term adhesion to plasticized vinyl surfaces, prepared from a representative blend of dioctyl phthalate plasticizer and a terpolymer of an alkyl acrylate, a nitrogen containing vinyl monomer and a vinyl carboxylic acid. The adhesive is designed for plasticized surfaces.
U.S. Pat. No. 4,364,972 (Moon, assigned to 3M Company) discloses a pressure-sensitive adhesive which adheres strongly to automotive paints. This adhesive consists essentially of the photopolymerized reaction product of an acrylic acid ester of a non-tertiary alcohol, the alkyl groups of which have an average of 4-14 carbon atoms, and N-vinyl-2-pyrrolidone in an amount within 15-50 parts by weight of the total monomers. According to Moon, if other modifying copolymerizable monomers are added to this pressure-sensitive adhesive composition, such as acrylic acid, acrylonitrile and N-substituted acrylamides, then a loss in "quick-stick" or tack of the adhesive would result.
U.S. Pat. No. 5,019,377 (assigned to Proctor and Gamble) discloses low glass transition adhesive compositions of an acrylic or methacrylic ester copolymer with acrylamides or methacrylamides. The adhesives are useful for hairstyling products, but do not have good pressure-sensitive adhesive properties. The acrylamide and methacrylamide monomers impart hydrophilicity to the formulation.